Thermoplastic urethane resins which have superior mechanical strength and wear-resisting properties are employed in the production of various articles such as hoses, belts, and coatings for electric cables, pipes and many other shaped articles. However, thermoplastic urethane resins are inflammable and cannot be used in applications where flame retardancy is required.
Articles having a thermal recovery property such as heat-shrinkable tubing are generally produced by the method in which a tube formed of a crosslinked crystalline polymer such as polyethylene is expanded to a predetermined diameter while heating to a temperature not higher than the melting point of the polymer, and the expanded tube is cooled intact to a temperature below the melting point while maintaining its shape (i.e., shape-memory effect). Details of this method are described in e.g., U.S. Pat. No. 3,086,242.
Polyurethane-based articles having such a thermal recovery property are known (e.g., described in U.S. Pat. No. 3,624,045) and one may try to fabricate heat-shrinkable tubing from thermoplastic urethane resins with a view to making use of their superior mechanical strength and wear-resisting properties. However, if urethane resins are directly heated to their heat deformation temperature (e.g., about 180.degree. C.) or higher, they will melt and this will cause difficulty when an attempt is made to expand them. In addition, unmodified urethane resins are not adapted to applications where flame retardancy is required.
While several methods have been proposed for improving such properties of high-molecular weight materials as heat resistance, crosslinking between molecules is general in the area of polyethylene. This method for crosslinking the molecules of a polymer is conventionally achieved by various methods, e.g., chemical crosslinking with an organic peroxide; radiation crosslinking such as with electron beams or gamma-rays; and water crosslinking with a reactive silane. However, the chemical crosslinking and water crosslinking methods are not completely suitable for the purpose of crosslinking thermoplastic urethane resins for the two principal reasons that the resins require a temperature of at least 180.degree. C. for molding into a desired shape, but this temperature is higher than the decomposition point of organic peroxides, and it is difficult to control accurately the addition reaction of reactive silane compounds.
The present inventors therefore made concerted efforts in order to crosslink thermoplastic urethane resins so as to render them suitable for use as a material for fabricating articles having a thermal recovery property, such as heat-shrinkable tubing. The present inventors also studied the possibility of imparting flame retardancy to thermoplastic urethane resins.